1. Field of the Invention
The present invention relates to an organic EL device having an organic luminescent layer that is formed by a relief printing method, a method for producing the same, and a relief printing plate used therefor.
2. Related Art
An organic EL device is a light emitting device that has a structure having an organic light emitting layer formed of an organic light emitting material between an anode and a cathode facing each other, and positive holes and electrons are injected to the organic light emitting layer from the anode and the cathode by applying a voltage, respectively, and are recombined therein to emit light. The thickness of the organic light emitting layer is an important factor for emitting light with high efficiency, and the thickness is necessarily controlled within a range of from several nm to several tens nm. In the case where an organic EL device is used as a display device, it is necessary to employ a technique for patterning the device with high definition.
The organic light emitting material used in the organic light emitting layer is classified into a low molecular weight material and a high molecular weight material, and the formation method of the organic light emitting layer varies depending on the kind of the organic light emitting material.
In general, the low molecular weight material is formed into a thin film on a substrate by a resistance heating vapor deposition method (vacuum vapor deposition method) or the like. In the case where a full color display device is produced with the organic EL device, light emitting materials with plural light emitting colors are vapor-deposited on pixel portions respectively with masks having patterns corresponding to the shapes of the pixels of the respective colors. This method is excellent for forming a uniform thin film but has a problem that the pattern accuracy of the mask is difficult to maintain on forming on a large substrate.
The organic light emitting material in the form of a high molecular weight material is generally dissolved or dispersed in a solvent to form an ink, which is formed into a thin film by a wet coating method. Examples of the wet coating method include a spin coating method, a bar coating method, an extrusion coating method and a dip coating method, but these methods are difficult to use for patterning or coating colors distinctly, and a pattern printing method, such as an ink-jet method and a printing method is necessarily employed for patterning with high definition.
For example, in a pattern forming method by an ink-jet method disclosed in JP-A-10-12377, an organic light emitting material dissolved in a solvent is ejected from an ink-jet nozzle onto a substrate and dried on the substrate to form a pattern. However, ink droplets ejected from the nozzle are in a spherical shape, and thus the ink spreads to a circular shape on impacting on the substrate, which may cause a problem that the pattern shape thus formed lacks linearity, or the pattern lacks linearity due to deteriorated impact accuracy.
Examples of the pattern formation method by a printing method include a relief printing method, an inversion printing method and a screen printing method. The organic EL device or a display often employs a glass substrate as a substrate thereof, for which a method using a hard printing plate, such as a metal printing plate, for example, a gravure printing method, is not suitable. Accordingly, such printing methods may be properly applied as a printing method using a rubber printing plate having elasticity, an offset printing method using a rubber printing blanket, and a relief printing method using a photosensitive resin printing plate formed mainly of rubber or other resins having elasticity. In practice, a pattern printing method by offset printing (JP-A-2001-93668), a pattern printing method by relief printing (JP-A-2001-155858) and the like have been proposed as attempts of the printing methods.
As an example of a process for forming an organic light emitting layer of an organic EL device by a relief printing method, an organic light emitting material is dissolved or dispersed in a solvent to form an ink, which is coated on a surface of an anilox roll having fine pores. An excessive portion of the ink on the surface of the anilox roll is scraped out with a doctor blade for uniformizing the coated amount of the ink per unit area of the anilox roll. The ink on the anilox roll is then transferred to an image forming part of a printing plate, in which the image forming part is provided corresponding to the shape of the pixels of the organic EL device, and finally the ink thin film on the image forming part of the printing plate is transferred onto the substrate to form the organic light emitting layer of the organic EL device.
FIG. 9 is a schematic cross sectional view showing an example of a printing process for forming a light emitting part by a relief printing method. In FIG. 9, an ink pan 101 stores an ink containing an organic light emitting material and the like dissolved or dispersed in a solvent. The ink is fed to an anilox roll 103 in the ink pan 101, and the excessive portion of the ink thereon is scraped out with a doctor blade 102, thereby accumulating the ink only inside the pores. The ink in the pores is transferred onto an image forming part of a relief printing plate 104 wound on a plate cylinder 105, which is rotated against the anilox roll 103, thereby forming a patterned ink thin film thereon. The patterned ink thin film formed on the image forming part is further transferred onto a substrate 106, which is moved against the plate cylinder 105, thereby forming an intended light emitting part at the position on the substrate where the light emitting part is to be formed.
In the case where an organic EL device is produced by a printing method, the pattern of the image forming part on the printing plate for forming a light emitting part contains plural image forming parts that form light emitting parts emitting light in the same color among plural light emitting parts constituting the respective elements of the organic EL device, and plural printing plates having such a structure are provided for each colors of light to be emitted, with which the light emitting parts emitting light in the colors respectively are sequentially printed, thereby providing the organic light emitting layer.
FIGS. 6A to 6D are schematic cross sectional views showing an ordinary printing process by a relief printing method, and the printing process is described with reference thereto. A printing plate is inked (FIG. 6A) and then pressed onto a transfer substrate 201 (FIG. 6B). At this time, a relief pattern 203 of the plate is pressed onto the transfer substrate, and the ink 202 thus squashed spreads outside the relief pattern of the plate. On removing the printing plate off from the transfer substrate, the ink is separated into the printing plate and the transfer substrate due to the viscous behavior of the ink (FIG. 6C). Thereafter, the ink thus transferred to the transfer substrate is leveled to form a uniform pattern (FIG. 6D). The printed line width of the pattern thus formed by the process tends to be thicker than the relief pattern width of the plate by such an extent that the ink on the relief pattern is squashed and spread from the relief pattern.
In the case where the ink thus transferred in the aforementioned printing process is not sufficiently leveled, printing defects may occur, such as a cupping phenomenon, in which the ink is not transferred to the center portion of the printed line, and a marginal phenomenon, in which the ink is not transferred to the outer portion of the printed line. When the phenomena occur, the organic material layer may have uneven thickness within a pixel, which causes light emission unevenness. The states of occurrence of the phenomena are shown in FIGS. 7A, 7B, 8A and 8B. FIGS. 7A and 7B are schematic cross sectional views showing the cupping phenomenon, and FIGS. 8A and 8B are schematic cross sectional views showing the marginal phenomenon.
These phenomena may occur in the case where the relief pattern of the plate has a large width, and the case where the ink has large viscosity. Even though the phenomena are not observed on printing for several sheets, they tend to occur on performing continuous printing.
Accordingly, for producing an organic EL device continuously by a relief printing method stably, the ordinary relief printing plate has problems in the size of the transfer area of the printed material and in the kind and physical properties of the ink.